Fungi and bacteria are ubiquitous microorganisms. Endophyte is the term first coined by de Bary [1866] defining those microbes that colonize asymptomatically plant tissues [Stone et al., 2000]. The existence of endophytes has been known for more than one century [Freeman 1904] and it seems that each individual host, among the 300,000 plant species, inhabits several to hundreds of endophytes [Tan and Zou, 2001]. Endophytes are microbial organisms mostly symbiotically or mutualistically associated with living tissues of plant hosts. Many are capable of conferring plant tolerance to abiotic stressors or can be used by the plant for defense against pathogenic fungi and bacteria [Singh et al. 2011]. Some of these microorganisms have proven useful for agriculture, forestry and horticulture sectors, as well as plant production of medicinally important compounds.
Endophytes largely determine plant cell and whole plant genome regulation, including plant's vital cycles: (i) seed pre- and post-germination events (mycovitalism) [Vujanovic and Vujanovic 2007], (ii) plant nutrient uptake and growth-promoting mechanisms (mycoheterotrophism) [Smith and Read 2008], and (iii) plant environmental stress tolerance and induced systemic resistance against diseases and pests (mycosymbionticism) [Wallin 1927; Margulis, 1991]. They could play a major role in plant biomass production, CO2 sequestration, and/or yield and therefore be significant players in regulating the ecosphere, ensuring plant health and food security. In addition, they can be important sentinels (bioindicators) of environmental changes, as alterations in the structure and biomass of endophytic communities can herald changes not only in pathways of nutrient (N, P, K), energy transfer in food-webs and biogeochemical cycles but also in UV-B, heat, drought or salt tolerance influencing the overall plant ecosystem establishment and stability. Despite their abundance and likely importance in all terrestrial ecosystems, nearly nothing about the composition of endophytes in seeds or spermosphere, their interactions, or their common response to environmental changes is known.
While the spermosphere represents a rapidly changing and microbiologically dynamic zone of soil surrounding a germinating seed [Nelson, 2004], the rhizosphere is a microbiologically active zone of the bulk soil surrounding the plant's roots [Smith and Read 2008]. The rhizosphere supports mycoheterotrophy or a plant-mycorrhiza symbiotic relationship. The spermosphere, on the other hand, promotes mycovitality or an endophytic fungi relationship with the plant seeds—enhancing seed vigour, energy and uniformity of germination that could be fairly predicted. Fungal endophytes are distinct from mycorrhizae in that they can colonize not only roots, but also other plant organs including seeds [Vujanovic et al. 2000; Hubbard et al. 2011]. They belong to the multicellular phyla Ascomycota and Basidiomycota and form colonization symbiotic structures different from those produced by unicellular or cenocytic phylum Glomeromycota, known as vesicular-arbuscular mycorrhizal symbiosis [Abdellatif et al. 2009]. Endophytic bacteria have been also found in virtually every plant studied, where they colonize an ecological niche similar to that of fungi, such as the internal healthy tissues. Although most bacterial endophytes appear to originate from the rhizosphere or phyllosphere; some may be transmitted through the seed [Ryan et al. 2008].
Seed germination is a vital phenophase to plants' survival and reproduction in either optimal or stressful environmental conditions. Microbial endophytic colonization at the seed state is especially critical because of the role of the seed as a generative organ in regeneration and dispersion of flowering plants [Baskin and Baskin 2004] and the role of mycobionts and symbiotically associated bacteria (bactobionts) as potential drivers of seedling recruitment in natural—undisturbed, disturbed and polluted—habitats [Mühlmann and Peintner 2000; Adriaensen et al. 2006; White and Torres 2010]. Thus, developing methods by which seedling emergence can be enhanced and protected under the limitations of disease pressure, heat or drought is precious. The use of endophytic symbionts is a promising method by which seed germination can be enhanced [Vujanovic et al. 2000; Vujanovic and Vujanovic 2006; Vujanovic and Vujanovic 2007]. It was hypothesized that plant stress hardiness can be conferred via a mycobiont-seed relationship known as mycovitality—a phenomenon that had been reserved for Orchidaceae [Vujanovic 2008] and via bactovitality which refers to a form of bactosymbiosis, using different endophytic strains with variety of activities.